Conventionally, in a narrow band excimer laser device and fluorine laser device, a slit for shaping a sectional form of laser light into a predetermined form is known, and it is disclosed, for example, in Japanese Patent No. 2531788. FIG. 12 shows a configuration of an excimer laser device according to the prior art. It should be noted that FIG. 12 is made by being reversed horizontally relative to the drawing made in the aforementioned Patent. In FIG. 12, the excimer laser device 1 is seen from above, and in the explanation hereinafter, an up and down direction of the paper surface of FIG. 12 is called a lateral direction, and a direction vertical to the paper surface is called a vertical direction.
In FIG. 12, an excimer laser device 1 includes a laser chamber 2 containing laser gas being a laser medium at a predetermined pressure ratio, and inside the laser chamber 2, discharge electrodes 5 and 5 are placed to oppose to each other in the aforementioned vertical direction. High voltage is applied across the aforementioned discharge electrodes 5 and 5 from a high voltage power supply not illustrated to initiate discharge, and thereby the laser medium is excited in a discharge area 18 to oscillate laser light 11.
The laser light 11 excited in the laser chamber 2 is outputted from a rear window 9 toward the rear (the left side in FIG. 12), and its bandwidth is narrowed by a grating 23 so that a spectrum width of the laser light 11 become narrow. The laser light 11 with its bandwidth being narrowed enters the laser chamber 2 again from the rear window 9, and is outputted from the excimer laser device 1 through a front window 7 and a front mirror 38 to become a light source for processing of a processing unit such as a stepper or the like not illustrated.
In this situation, partial reflection coating for partially reflecting part of the laser light 11 at a predetermined ratio and transmitting and outputting the rest of the laser light 11 is applied on an entire surface of the front mirror 38 to the side of the laser chamber 2, and defines a partial reflecting element 26. The laser light 11 which is partially reflected by the front mirror 38 returns to the inside of the laser chamber 2, and is amplified again by discharge in the discharge area 18.
In front of and behind the laser chamber 2, placed are a front slit 16 and a rear slit 17 (described as “aperture” in the aforementioned Japanese Patent No. 2531788) having a rectangular front opening 16A and rear opening 17A respectively. In the above prior art, the front slit 16 and the rear slit 17 correspond to optical elements for shaping the beam form of the laser light 11 into a desired form.
In the excimer laser device 1, part of the laser light 11 is cut by the aforementioned slits 16 and 17, and the sectional form of the laser light 11 is shaped into the form of the openings 16A and 17A so that the sectional form of the laser light 11 corresponds to a form required by a processing unit. Hereinafter, the sectional form of the laser light 11 is called a beam form.
The rear slit 17 shapes the beam form of the laser light 11 traveling rearward, thereby preventing disturbance of the wave surface caused by the laser light 11 hitting an end portion of the grating 23.
However the aforementioned prior art has the disadvantage described below.
FIG. 13 shows a view taken along the 13—13 line in FIG. 12. It should be noted that the aforementioned lateral direction is represented as the left and right direction in FIG. 13. The illustration of the front window 7 is omitted. FIG. 14 shows a detailed configuration of the area near the front and the rear windows 7 and 9 of the excimer laser device 1.
As shown in FIG. 13, both the front opening 16A and the rear opening 17A according to the prior art are narrower than the discharge area 18 in which the laser medium is excited. As a result, even if the shape of the discharge area 18 is varied as a result of consumption of the discharge electrodes 5 and 5, the laser light 11 passing through the openings 16A and 17A can obtain a stable beam form.
However, as a result that the openings 16A and 17A are made narrower than the discharge area 18, as shown in FIG. 14, surplus laser light 11A, which is oscillated at the outer peripheral side of the discharge area 18 than the openings 16A and 17A, is cut by the slits 16 an 17. Thus, out of the discharge energy inputted into the discharge area 18, a part of it becomes a loss such as heat or the like and is not taken out as the laser light 1, which causes the disadvantage of reducing the efficiency of the excimer laser device 1.
Further, as shown in FIG. 14, the surplus laser light 11A is outputted to areas close to the openings 16A and 17A of the slits 16 and 17. Thus, the temperature in the areas close to the openings 16A and 17A of the slits 16 and 17 rise and thereby refractive index of the gas inside the openings 16A and 17A is varied, thus causing the disadvantage of the wave surface of the laser light 11 being disturbed. Furthermore, heat occurs in the areas close to the openings 16A and 17A of the slits 16 and 17 causes the slits 16 and 17 to have heat, which causes the disadvantage that impurities occurring there stain and damage the other optical components.
Further, in the rear slit 17, as shown in FIG. 14, the rear opening 17A is made smaller than the front opening 16A. Thereby, a part 11C of the laser light 11, which is partially reflected by the front mirror 38, passes through the front opening 16A, and returns to the discharge area 18, cannot pass through the rear opening 17A and is cut, thus further increasing the loss.